src: add missing namespace using statements in node_watchdog.h

Although these `using`s can derived from other header files, it will
be better to be self-contained.

PR-URL: https://github.com/nodejs/node/pull/32117


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>